Not only "coke bugs," but "meth bugs,"speed bugs," — the overuse of any stimulant will cause the same sensation. With cocaine and methamphetamine, the phenomenon occurs when the body can’t digest the hazardous additives. It instead forces them out through the pores, causing sores, acne and chronic scratching. This phenomenon was first noted in the 1890's..and in all decades subsequently. It is also related to over-stimulation of the nerve endings.
1902 Atlanta Constitution(Ga.) (Nov. 29) “Opium, Cocaine and Whiskey Habits and Their Baleful Effects” p. E4: There are many strange things connected with the drug habit and the “cocaine bug” is one of the most singular. The victim imagines there is a bug crawling about just under his skin.
This has been studied by experts, and it turns out that this "condition" is almost universal when it comes to overuse of stimulants.
'In humans, the probing forefinger and pincer grasp is also directed (‘in grooming behaviors') at raised follicles or small irregularities in the skin, resulting in numerous punctuate lesions and scars. The associated cognitive manifestation of this stereotype attitude is the development of delusions of parasitosis." – Chronic Amphetamine Use and Abuse Everett H. Ellinwood, M.D., George King, Ph.D.and Tong H. Lee, M.D. Ph.D.
So, if you use stimulants, and you think that you have bugs — you have simply taken too many stimulants. Drink plenty of water, don't use any more stimulants than your are medically prescribed. If you are using stimulants for fun, recognize that the sensation that you have bugs is indicitive of overuse. You don't want to end up like those folks who rip their flesh apart trying to get the little critters.